Generally, an optical amplifier that employs an optical fiber doped with a rare earth (hereinafter, “rare earth-doped fiber”) as an amplification medium is used as an optical amplification repeater adopted in a WDM transmission system. This optical amplifier using the rare earth-doped fiber is an amplifier that can amplify all the optical signals in an amplification band at once as they are, and plays the role of an engine of stimulating the WDM technology because of its convenience and facilitation.
Meanwhile, for an optical amplifier used in the WDM transmission system, evenness in the amplification gain is required, with which the wavelength level of each signal light can be amplified in a substantially stable manner over the entire amplification band. With the above described optical amplifier, however, once the light intensity of an input signal input into the optical amplifier is changed due to a change in the signal wavelength, variation in the loss in the transmission channel, or the like, the gain profile of the optical amplifier changes, which results in throwing the optimum signal level diagram of the transmission system out of balance. This may reduce a ratio of signal to noise intensity (SNR). In addition, the quality of the transmission system affected by a non-linear transmission channel fiber may be significantly degraded.
Among techniques for suppressing the quality degradation of such a transmission system, a constant gain control technique of controlling the gain of an optical amplifier to stabilize, regardless of the input signal wave number (intensity), is disclosed (for example, Patent Document 1).
With the constant gain control technique disclosed in the Patent Document 1, in the process of detecting the gain of a forward-pumping optical amplifier using a rare earth-doped fiber from the input signal light intensity (Pin) and the output signal light intensity (Pout), PASE is input as a constant (≠0) that corresponds to the light intensity of amplified spontaneous emission (ASE) caused in the rare earth-doped fiber, and (Pout−PASE)/Pin is defined as the gain so that the power of the forward-pumping light is controlled to keep this gain constant. In other words, with the conventional optical amplification control technology, an error is compensated in the constant gain control, using the light intensity of spontaneous emission contained in the full output light of a light amplifier as a constant.
Patent Document 1
Japanese Patent Application Laid-open No. 11-112434 (p. 8, FIG. 1)
The above compensation technology (constant gain control technology), however, is based on the assumption that a rare earth-doped fiber has an ideal property of evenness, which prevents a gain profile variation component generated by the unevenness of the rare earth-doped fiber from being compensated. Thus, the gain profile variation caused in accordance with changes in the input signal light level, the number of wavelengths, and the wavelength results in a control error, creating a problem that the dynamic range of the input signal light, which is to maintain the evenness of the gain of the optical amplifier, is narrowed. There is also a problem that it is not applicable to certain types of rare earth-doped fibers in which a gain profile variation resulting from the unevenness is pronounced.
The present invention has been conceived in consideration of the above situation. The primary object of the present is to provide an optical amplifier that compensates a gain profile variation resulting from the unevenness of the optical amplifier. Then, the secondary purpose of the present invention is to disclose a gain control method for compensating a gain profile variation resulting from the unevenness of the optical amplifier and constitute the variable gain control method in a simple and accurate fashion.